In a known drum brake of this type (DE 3021794 A1) the intermediate support is mounted for oscillation or is displaceably guided on the anchor plate in such a manner that the ends of the two brake shoes supported thereon can execute a floating movement substantially parallel to the wheel brake cylinder. When this brake is mechanically actuated whilst the associated vehicle is travelling forwards or for example tends to roll forwards due to a gradient, the brake drum entrains the first brake shoe so that the end thereof normally supported at a closed end face of the wheel brake cylinder moves away from the latter and the other end thereof exerts a peripheral force on the adjacent end of tile second brake shoe via the floating intermediate support. The second brake shoe is pressed by this force and by peripheral forces acting from the brake drum directly thereon against the piston of the wheel brake cylinder so that the latter is displaced into the wheel brake cylinder and displaces brake fluid until it comes to bear on a stop in the wheel brake cylinder.
The displaced brake fluid must be replenished by the master brake cylinder on the next hydraulic brake actuation. This requires a travel of the brake pedal which is greater than the usual pedal travel. In unfavourable cases, the travel of the brake pedal necessary for refilling the wheel brake cylinders can be greater than the maximum possible pedal travel so that adequate hydraulic braking is possible only after repeated actuation of the brake pedal, if at all. The known drum brake thus did not acquire any practical significance although in a desirable manner it has a braking coefficient which in hydraulic braking is comparatively small and has a degressive characteristic but on mechanical actuation is comparatively large and has a progressive characteristic.
The braking coefficient is understood as the ratio of the sum of the peripheral forces acting on the brake shoes during braking to tile clamping force with which the brake shoes are pressed against the brake drum. Characteristic means the dependence of the brake coefficient on the friction coefficients of the pair comprising brake lining and brake drum.